Internal combustion engines include a piston that is slideably moveable within a cylinder bore of an engine block. The piston moves in a reciprocating motion through at least a compression stroke, having a compression stroke length, and an expansion stroke having an expansion stroke length. In a standard Otto cycle internal combustion engine, a connecting rod is eccentrically connected to a crankshaft relative to a crank axis. The connecting rod interconnects the piston and the crankshaft. Rotation of the crankshaft moves the piston through its compression and expansion strokes, with the compression stroke length being equal to the expansion stroke length.
An Atkinson cycle internal combustion engine uses a system of linkages to interconnect the piston and the crankshaft to a parallel control shaft. The unique linkage system of the Atkinson cycle engine enables the compression stroke length to be less than the expansion stroke length. By reducing the compression stroke length relative to the expansion stroke length, the compression ratio is less than the expansion ratio. This increases the fuel economy of the engine during some operating conditions.
A compression ratio is one of the fundamental specifications of an internal combustion engine. An internal combustion engine's compression ratio is a value that represents the ratio of the volume of the engine's combustion chamber from its largest capacity to its smallest capacity. In a reciprocating internal combustion engine, the compression ratio is typically defined as the ratio between the volume of the cylinder and combustion chamber when the piston is at a bottom of its compression stroke, and the volume of the combustion chamber when the piston is at a top of its compression stroke.
A more modern variation of the Atkinson cycle internal combustion engine is a Miller cycle internal combustion engine. The Miller cycle engine uses valve timing to achieve the results provided by the linkage system of the Atkinson cycle engine, i.e., the compression stroke length being shorter than the expansion stroke length. The Miller cycle engine uses valve timing to hold open the intake valve during the initial phases of the compression stroke, thereby effectively shortening the effective compression stroke length. The use of Miller cycle internal combustion engines is gaining popularity due to potential fuel economy gains. At low loads and low engine speeds, Miller cycle engines provide significant fuel economy savings, especially when combined with a high compression ratio to take advantage of thermal efficiency gains. However, at high loads and high engine speeds, Miller cycle engines with a high compression ratio become extremely spark limited, to a point where the maximum power potential of the engine is unachievable.